1. Field of the Invention
The invention concerns the production of highly wear-resistant ledeburitic surface layers of cast-iron machine components. The present invention is useful in all cast-iron components subject to wear as a result of lubricated friction. The invention is particularly advantageous for use in the production of engine components, such as camshafts, cam followers, rocker arms, cylinder liners, or the like.
2. Discussion of Background Information
Ledeburitic surface layers have very good wear resistance to sliding friction under hydrodynamic or mixed friction conditions.
It is known to produce such layers for camshafts by TIG remelting (e.g., Heck: Influence of Process Control in Remelt Chilling on the Surface Layer Properties of Camshafts Made of Ledeburitic Cast Iron, Dissertation, Munich, 1983). For this, a TIG burner is guided relatively slowly at approximately 125 to 225 mm/min at a right angle to the feed direction with a low oscillation frequency of approximately 0.7 to 2.2 Hz in pendulum fashion along the camshaft circumference. The power density used is roughly 3000 W/cm2. Thus, heating speeds of approximately 200-750 K/s are achieved. In order to avoid cracks, preheating to temperatures of approximately 400xc2x0 C. is used.
The cams produced in this fashion have a coarse solidification structure which consists of relatively coarse ledeburitic cementite and pearlite in the metal matrix. Moreover, tempered zones are generated which are characterized by unfavorable damage to the remelted structure because of repeated temperature loading as a result of the slow pendulum action of the TIG burner.
A disadvantageous effect with cams produced in this manner is the fact that wear resistance is too low. The cause of the low wear resistance lies in the coarse grain structure and the additional coarsening of the structure within the tempered zones.
The major shortcoming of the method is that the solidification speed is too slow. The cause for this consists in the power density is too low, which makes it necessary to work with relatively low feed rates.
To counter this shortcoming, it is known to also use modern high-energy surface layer remelting methods such as laser beam remelting (e.g.: M. S. Mordike: xe2x80x9cPrinciples and Application of Laser Surface Refinement of Metalsxe2x80x9d, Dissertation, Clausthal-Zellerfeld, 1991; Patent DE 42 37 484) or electron beam remelting (e.g., Patent DE 43 09 870) for ledeburitic remelting of camshafts. For this, an appropriately shaped energy beam (e.g., rectangular; two rectangular radiation fields separated in the feed direction; scanning spot grids; grids with different power densities) with a feed rate which is constant or a function of the local radius curvature is guided over the camshaft such that one melting pool extending over the entire width of the cam is created, or a plurality of melting pools extending only slightly in the feed direction. Here, power densities of 103 to 105 W/cm2 are used. The feed rates are 500 to 2500 mm/min. To avoid cracks in the melt zones, it seemed indispensable to use intensive preheating to temperatures of approximately 360 to 550xc2x0 C. This occurs as a rule in expensive through-type furnaces.
The remelted cam regions have a remelted zone 0.3 mm to an average of approximately 0.8 mm deep. The remelted zone includes ledeburitic cementite and pearlite in the metallic matrix. When the austenitizing temperature is exceeded in the zone directly below the remelted zone, a new pearlitic zone of slightly higher hardness than that of the starting state is formed because of the slow cooling. The drop in hardness begins, consequently, immediately at the edge of the remelted zone and is relatively steep.
The shortcoming of cams produced in this manner is that they do not achieve the actual wear resistance possible for such a finely dispersed structural formation of the ledeburitic cementite. The reason for this is that the pearlite in the metallic matrix has lower wear resistance than the cementite and, consequently, represents the weak point of structure.
The shortcoming of the method is that pearlite develops both within the remelted zone and in the underlying new austenitizing zone. The cause for this is that, due to the high preheating temperatures of 360xc2x0 C. to 550xc2x0 C., the cooling speed in the temperature range of approximately 600xc2x0 C. to 450xc2x0 C. is already so low despite the high solidification speed that the residual austenite breaks down completely to form relatively coarse pearlite.
However, an optimum surface layer structure for wear resistance requires a layer structure consisting of a thin surface layer which is capable of accommodating the adhesive stresses occurring with tribologic loading, plastic deformations, and cyclic elastic-plastic microstrains, and an underlying support layer which accommodates the strains as a result of Herzian stresses. Consequently, an additonal shortcoming of this method is that this support layer can also only be formed by a remelted layer. The greater remelting depth necessary for this results in economic disadvantages due to the low feed rate required.
A cam with a surface layer structure better suited for wear resistance became known with patent EP 0 161 624. The cam surface layer includes a cementite layer with a large proportion of cementite and, under it, a martensitic layer, whereby the remelted layer has a depth of 0.3 to 1.5 mm and the underlying hardening zone has a thickness of 0.3 to 2.0 mm.
In this method, the cams, without preheating are brought to melting by a TIG arc and then solidify by self-quenching. In a subsequent patent (EP 0 194 506), to accelerate the cooling, water or a water air mixture is passed through the central oil bore in the lengthwise axis of the camshaft.
It is possible, without consequences for crack formation, to do without preheating, since the work is performed with a very low power of 1360-2600 W at very low rotational speeds of 0.7 to 1.0 rpm. This corresponds approximately to feed rates of 80 to 130 mm/min. At these slow feed rates, the heat introduced runs in front of the remelting spot and also penetrates very deeply into the cam during the remelting. Thus, the quenching speed is reduced so much that the crack formation stress is no longer reached during cooling. However, because of the low feed rate, the solidification speed is also reduced, which results in a coarser formation of the ledeburitic cementite compared to laser or electron beam remelted cams.
Despite the low cooling speed, cams treated in this manner have improved wear resistance compared with the TIG remelted cams with preheating. The only reason for this can be that the pearlite formed in the metallic matrix is clearly more finely laminated because of the higher cooling speed during its creation. The potential of possible improvement of properties due to a finely dispersed cementite formation can, however, not be realized.
Consequently, the shortcoming of cams produced in this manner is that they have no wear-optimal surface layers. The cause of this is the relatively coarse formation of the solidification structure as a result of the low solidification speed and the formation of tempered zones.
The low power density and slow feed rate result in a solidification speed too low for the formation of a finely dispersed structure. Another disadvantage is that the structure is macroscopically non-homogeneous and periodically has even coarser grain structures. The reason is the repeated local temperature exposure of already greatly cooled regions to far above the austenitizing temperature as a result of the vary low oscillation motion of the TIG burner.
The present invention provides a camshaft better protected against wear caused by sliding friction as well as a method for production thereof.
The invention further reports formation of a grain structure and a surface layer structure for camshafts and similarly loaded cast-iron components which are better suited to the use conditions of sliding friction loads with high load stresses under hydrodynamic or mixed friction conditions. Also, a method provided which works to establish finely dispersed structures with high power densities, avoids crack formation even without volume preheating, and at the same time essentially suppresses the formation of coarse pearlite by a relatively high cooling speed between 600xc2x0 C. and 350xc2x0 C.
According to the invention, a wear resistant cast-iron camshaft, is provided whose surface layer includes a ledeburitic remelted layer with a high cementite proportion and a martensitic hardening zone underlying it.
The remelted layer includes finely dispersed ledeburitic cementite with thicknesses xe2x89xa61 xcexcm and a metallic matrix of a phase mixture of martensite and/or bainite, residual austenite as well as less than 20% of finely laminated pearlite with a distance of  less than 0.1 xcexcm between lamellas. The underlying hardening layer includes a phase mixture of martensite and/or bainite, paritally dissolved pearlite, as well as residual austenite. The remelted layer can have a depth ts of 0.25 mmxe2x89xa6tsxe2x89xa60.8 mm and the hardening layer can have a depth of 0.5 mmxe2x89xa6tsxe2x89xa61.5 mm.
The depths ts of the remelted layer of the present invention are somewhat smaller than known in the prior art and thus use the supporting effect of the underlying layer in an economically advantageous manner.
In addition, the present invention provides a method for production of the wear-resistant camshaft using a high-energy remelting method. The method produces a wear-resistant camshaft by a high-energy surface remelting process. A temperature time curve of the remelting includes two superposed short-time temperature time cycles T1 and T2, which are generated with two different energy sources S1 and S2 with different power densities p1 and P2. The temperature time cycle T1 has a peak temperature T1max of 560xc2x0 C.xe2x89xa6T1maxxe2x89xa6980xc2x0 C., a heating time of 0.5 sxe2x89xa6t1xe2x89xa66 s, an average heating speed of (xcex94T1maxc/xcex94t1c) of 90 K/sxe2x89xa6(xcex94T1maxc/xcex94t1c)xe2x89xa61900 K/s and an initial quenching speed (xcex94T1a/xcex94t1a) of 50 K/sxe2x89xa6(xcex94T1a/xcex94t1a)xe2x89xa6500 K/s and the power density p1 of the energy source S1 reaches the value of 8xc3x97102 W/cm2xe2x89xa6p1xe2x89xa68xc3x97103 W/cm2. The temperature time cycle T2 has a peak temperature T2max of T2maxxe2x89xa7Ts, whereby Ts represents the melting temperature of the cast-iron used, an average heating speed (xcex94T2maxc/xcex94t2c) of 3000 K/sxe2x89xa6(xcex94T2maxc/xcex94t2c)xe2x89xa640,000 K/s, a solidification speed vs of the melt of 10 mm/sxe2x89xa6vsxe2x89xa667 mm/s as well as a power density p2 of the energy source S2 of 0.8xc3x97104 W/cm2xe2x89xa6p2xe2x89xa68xc3x97104 W/cm2 is selected. The time period t21=t2xe2x88x92t1 after the temperature time cycle T2 begins is 0.3 sxe2x89xa6t21xe2x89xa611 s. The temperature T1min at which the temperature time cycle begins is T1min greater than 500xc2x0 C.; the melting pool life ts is in the range of values from 0.08 sxe2x89xa6xcex94tsxe2x89xa60.8 s; and the feed rate vB of the high-energy energy source S2 reaches the value of 600 mm/minxe2x89xa6vBxe2x89xa64000 mm/min.
The entire width of the camshaft can be melted in one rotation. The necessary power density distribution p2 may be generated at a right angle to the feed direction by a rapid beam oscillation, in which the oscillation frequency is at least 200 Hz. The high-energy energy source S2 can be a laser. The rapid beam oscillation can include a rapid temporal and periodic sequence of a plurality of harmonic oscillation packets of different frequency f, amplitude A, center position Ao, and periodicity np. In this manner the number of different oscillation packets may be between 1 and 8, and periodicity is selected at 1xe2x89xa6npxe2x89xa620. The energy source S1 can be a medium-frequency induction generator. The high-energy energy source S2 can be an electron beam.
In accordance with the exemplary embodiments, the energy source S1 can be an electron beam. The high-energy energy source S2 may be high-performance diode laser. The energy source S1 may also be a high-performance diode laser. Further, the energy source S1 may include a plurality of high-performance diode lasers arranged in rotational symmetry around the camshaft and the camshaft can be preheated in the stationary process. Cementite stabilizing elements may be added to the melt in the casting of the camshaft. Austenite stabilizing elements may be added to the melt in the casting of the camshaft. Still further, cementite and/or austenite stabilizing elements can be added to the melt during the surface layer remelting with the high-energy energy source S2.
By the superposing of two short-time temperature cycles T1 and T2 it is possible to solve the contradiction which has existed previously the requirement for a high solidification and quenching speed as well as a relatively high and adjustable cooling speed between 600xc2x0 C. and 350xc2x0 C., on the one hand and the requirement for a low cooling speed below approximately 300xc2x0 C.
Thus, on the one hand, a finely dispersed solidification structure as well as a finely dispersed cycle of solid transformations with a controllable and relatively strong suppression of the formation of coarse pearlite is possible. On the other hand, the cooling speed in the crack-critical temperature range is adequately low to avoid cracks.
An advantage of some embodiments of the present invention is the fact that tempered zones as a result of excessive temperature fluctuations during remelting can be avoided.
An advantage of other embodiments of the present invention is the fact that because of a fast beam oscillation, the dimensions of the energy beam in the feed direction and perpendicular thereto can be set relatively flexibly and independent of each other and that with the oscillation frequencies reported, the temperature oscillations are small enough to prevent tempered zones. Thus, even with wide camshafts, short melting pool lives can be achieved.
Another advantage of embodiments of the present invention is the fact that power density distribution of the energy beam can be adapted to the heat dissipation conditions which differ toward the edge of the cam and the effects of the surface tension of the melt.
In accordance with the present invention, favorable energy sources for S1 and S2 include laser, medium-frequency induction generator, electron beam, high-performance diode laser, and a plurality of high-performance diode lasers arranged in rotational symmetry around the camshaft.
Another advantage of the present invention is the fact that with relatively slight changes of the chemical composition of the cast iron, the structure formation essential for the sliding friction wear characteristics can clearly be altered.
Another advantage of the present invention is that these slight changes in the chemical composition can also be integrated into the process.